Engine manufacturers have focused on improving engine design in order to minimise emissions of particulates, emissions of other pollutants, and also improve cleanliness, fuel economy and efficiency. One of the improvements in engine design is the use of exhaust gas recirculation (EGR) engines. Whilst improvements in engine design and operation have contributed to reducing emissions, some engine design advances are believed to have generated other challenges for the lubricant. For example, EGR is believed to have led to increased formation and/or accumulation of soot and sludge.
Soot-mediated oil thickening is common in heavy duty diesel engines. Some diesel engines employ EGR. The soot formed in an EGR engine has different structures and causes increased viscosity of engine lubricant at lower soot levels than formation of soot in the engine without an EGR. Attempts to alleviate soot-mediated oil thickening are disclosed in the references summarised below.
Traditional dispersant viscosity modifiers (DVMs) made from ethylene-propylene copolymers that have been radically grafted with maleic anhydride and reacted with various amines have shown desirable performance to prevent oil thickening in diesel engines. Aromatic amines are said to show good performance in this regard. DVMs of this type are disclosed in, for instance, U.S. Pat. Nos. 4,863,623; 6,107,257; 6,107,258; and 6,117,825.
U.S. Pat. No. 4,863,623 discloses controlling EGR soot by utilising maleic anhydride grafted ethylene-propylene copolymers capped with aromatic amines, such as 4-aminodiphenylamine.
U.S. Pat. No. 5,409,623 discloses functionalised graft copolymers as viscosity index improvers, containing an ethylene alpha-monoolefin copolymer grafted with an ethylenically unsaturated carboxylic acid material and derivatised with an azo-containing aromatic amine compound.
U.S. Pat. No. 5,356,999 discloses multifunctional viscosity index improvers for lubricating oils containing a polymer onto which has been grafted an unsaturated reactive monomer and thereafter reacted with amines containing sulphonamide units. The polymer is either an ethylene-propylene copolymer or an ethylene-propylene-diene terpolymer.
U.S. Pat. No. 5,264,140 discloses an ethylene alpha-monoolefin copolymer grafted with an ethylenically unsaturated carboxylic acid derivatised with an amide-containing aromatic amine material.
International publication WO 06/015130 discloses maleic anhydride grafted ethylene-propylene copolymers capped with sulphonamides, nitroanilines, diaromatic diazocompounds, anilides or phenoxyanilides. The copolymers are useful for controlling EGR soot.
Other dispersant viscosity modifying polymers suitable for lubricants have been contemplated including polyacrylic copolymers, including the disclosure of British Patent GB 768 701.
U.S. Pat. No. 4,234,435 discloses a composition in which a succinated polybutene is condensed with either an alkyl polyamine to make a succinimide dispersant or an alkyl polyol to make a succinic ester dispersant.
U.S. Pat. No. 5,182,041 discloses an additive composition comprising a graft and amine-derivatised polymer having an average molecular weight ranging from about 300 to 3500.
U.S. Pat. No. 7,361,629 and US Patent Application 2008/0171678 both disclose an amination product of a hydrocarbyl substituted succinic acylating agent and a mixture containing an aliphatic polyamine and an aromatic polyamine. The molar ratio of aliphatic polyamine to aromatic polyamine in the mixture ranges from about 10:0.1 to about 0.1:10.
PCT Application PCT U.S. Ser. No. 08/082,944 discloses an isatoic anhydride derived additive for reducing soot-mediated oil thickening and/or sludge formation.
Many of the attempts to alleviate soot-mediated oil thickening are believed to have a detrimental impact on seal performance (for example tensile strength and rupture elongation). The reason is because a number of known lubricant additives often deteriorate resin or rubber seals although they do satisfy lubricating performance requirements. The lubricant additives are believed to be sufficiently reactive that they shrink the seals and/or impair their strength and elasticity. Examples of the seals include resin or rubber seals such as silicone rubber seal, acrylic rubber seal, fluorocarbon resin seal, nitrile rubber seal, hydrogenated nitrile rubber seal and ethylene-propylene rubber seal.